Orthogonal Frequency Division Multiple Access OFDMA is a multiple access technique that has started to gain attention in wireless radio access systems. For example, the IEEE 802.16e standard (also known as the WiMAX (Worldwide Interoperability for Microwave Access) standard) utilizes the OFDMA for its physical layer both in downlink and uplink. Also the future evolutions of 3G (third generation) cellular systems, such as 3.9G or 4G systems, are presumably adopting OFDMA, at least for the downlink part. A channel allocation in an OFDMA system has both a time dimension and a frequency dimension. The smallest possible channel allocation unit is called a slot referring to a contiguous block of m logical sub-channels and n OFDM (Orthogonal Frequency Division Multiplexing) symbols, where m and n are integers. In the OFDMA, a single OFDM symbol may contain transmissions to/from several mobile stations. A logical sub-channel may contain several physical sub-carriers (which do not necessarily have to be adjacent to each other). In the OFDMA the possibility of dynamically dividing a single OFDM symbol between several terminals gives more freedom for channel allocation compared e.g. to normal OFDM based systems where each (unicast) symbol is dedicated to one terminal.
However, this freedom of the OFDMA also has drawbacks. For example, a terminal has to be able to identify the slots reserved for it. Therefore, a so-called map message is included in each transmitted frame in order to inform the terminals of the allocations related to them. The large size of the map messages has become one of the bottlenecks of the OFDMA-based systems, such as WiMAX or 3.9G systems. This problem is emphasized particularly if there are a lot of terminals that require small but steady throughput. Examples of such terminals include the ones using VoIP (Voice over IP (Internet Protocol)) services. In some cases, the map messages may even take tens of percents of the frame capacity, thus significantly reducing the available system capacity. In addition, the larger the map message becomes, the more likely it is that errors occur in its reception. If a terminal is not able to receive map messages, it will not be able to receive/send any data. In other words, large map messages result in a lowered spectral efficiency and higher packet error rates. In the WiMAX, these allocations are grouped into bursts, and the location of the bursts is then identified in the map messages sent at the beginning of each physical frame.
One possibility of reducing the size of the map message is to define a static allocation of slots. For example, at the beginning of a VoIP call, it can be agreed that certain slots are always allocated to the terminal with a certain sequence (for example, first two slots in every third frame). However, that kind of solution is not able to adjust to changes, such as silent periods in the VoIP call or variations in channel quality.